1. Field of the Invention
The present invention relates to a semiconductor laser driving apparatus, a semiconductor laser driving method, and an image-forming apparatus, particularly to a semiconductor laser driving apparatus, semiconductor laser driving method, and image-forming apparatus capable of preventing mode-hopping noise from being generated by a semiconductor laser.
2. Related Art
Optical scanning for recording various pieces of information including image information by scanning a recording medium with an optical beam or for reading various pieces of information from a recording medium uses a laser beam emitted from a semiconductor laser as an optical beam for scanning.
FIG. 14 shows functional blocks of a conventional semiconductor laser driving apparatus used to perform optical scanning with a laser beam emitted from a semiconductor laser. As shown in FIG. 14, the semiconductor laser driving apparatus is provided with semiconductor lasers of three types, for emitting laser beams having wavelengths different from each other, and the laser beam emitted from each semiconductor laser is modulated by a modulating section in accordance with image data.
Moreover, in this semiconductor laser driving apparatus, the light amount of a laser beam emitted from each semiconductor laser is detected by a light amount detecting section and a semiconductor laser driving section is controlled for APC (Auto Power Control) by a light amount control section so that the detected light amount becomes equal to a predetermined target light amount.
Furthermore, in this semiconductor laser driving apparatus, similarly to the aforementioned APC, the temperature at or near a light-emitting section of each semiconductor laser (hereafter referred to as semiconductor laser temperature) is detected by a temperature detecting section, and a temperature adjusting section is controlled by a temperature control section for ATC (Auto Temperature Control) so that the detected temperature becomes equal to a previously stored setting temperature (control temperature).
When performing optical scanning by a laser beam emitted from a semiconductor laser, it is generally required that a maximum fluctuation of the light amount of the laser beam is kept at approximately 1% or less when the optical scanning administers, for example, continuous gradation image information. That is, if the maximum fluctuation exceeds approximately 1%, visible irreguralities occur in a recorded image or in an image formed from image information read by scanning.
However, mode-hopping noise may be generated in the semiconductor laser, depending on driving conditions. That is, different modes (laser wavelengths) compete with each other in the semiconductor laser under certain driving conditions, mode-hopping from one to another of the different modes, and from the other to the one, is reciprocatingly repeated, and the light amount of the laser beam fluctuates due to the repeted mode-hopping. That is, mode-hopping noise is generated.
The fluctuation of a light amount due to the mode-hopping noise reaches several percent of total light amount. Therefore, when mode-hopping noise is generated during optical scanning for administering continuous gradation image information, extreme irregularities occur in the recorded image or the image formed from read image information. Moreover, mode-hopping noise fluctuates not only light amounts but also wavelength, which may affect images.
Therefore, to drive a semiconductor laser for optical scanning for handling continuous gradation image information, it is necessary to drive the semiconductor laser such that mode-hopping noise is not generated. By preventing mode-hopping noise, neither light amount nor wavelength fluctuate and a preferable image can be obtained.
Techniques for preventing mode-hopping noise from being generated are disclosed in Japanese Patent Application Laid-Open (JP-A) Nos. 61-260691, 61-260693, 61-264773, and 61-264774.
Each of the techniques disclosed in JP-A Nos. 61-260691 and 61-260693 stabilizes the wavelength of a laser beam to a single longitudinal mode by superimposing on a driving signal to be applied to a semiconductor laser high-frequency pulses having a frequency higher than that of the driving signal and adjusting at least one of parameters such as frequency, duty ratio, and amplitude of the high-frequency pulses, such as frequency, duty ratio, and amplitude. Thus, it is possible to prevent mode-hopping noise from being generated.
Further, a technique disclosed in JP-A No. 61-264773 uses a detector for detecting mode-hopping noise to change either or both of a setting light amount and a setting temperature when mode-hopping noise is detected by the detector.
Furthermore, a technique disclosed in JP-A No. 61-264774 measures and stores a temperature region in which mode-hopping noise is generated in advance, and controls a combination of light amount and temperature of a semiconductor laser such that the combination does not move to that temperature region. Thus, it is possible to prevent mode-hopping noise in advance.
However, in the case of the techniques disclosed in the above JP-A Nos. 61-260691 and 61-260693, it is necessary to superimpose high-frequency pulses on the semiconductor laser driving signal as described above. Therefore, there is a problem that cannot be prevented in that the apparatus is increased in size because hardware and control are complicated, and cost increases. Moreover, these techniques have a problem in that image quality is deteriorated because superimposed high-frequency pulses may function as noise when forming an image.
Moreover, the technique disclosed in JP-A No. 61-264773 has a problem in that it is impossible to completely prevent mode-hopping noise from being generated because generation of mode-hopping noise is detected by a detector and thereafter one or both of the setting light amount and the setting temperature is changed and thus, mode-hopping noise is continuously generated until the change is completed. Moreover, this technique requires a detector for detecting mode-hopping noise and thus, there is a problem in that cost increases.
Moreover, the technique disclosed in JP-A No. 61-264774 has a problem in that it is impossible to absorb shifts to temperatures at which mode-hopping noise is generated, due to various conditions of actual optical scanning (such as laser-beam emission cycle and emission start timing of the semiconductor laser) because temperatures at which mode-hopping noise is generated are not measured in conditions of actual optical scanning, and it is impossible to completely prevent the generation of mode-hopping noise. Moreover, because the technique requires a special apparatus for measuring temperatures at which mode-hopping noise is generated, there is a problem in that cost increases.
The present invention is provided to solve the above problems and an object of the present invention is to provide a semiconductor laser driving apparatus and a semiconductor laser driving method which can be made compact and fabricated at low cost and which make it possible to assuredly prevent the influence of mode-hopping noise and, moreover, to provide an image-forming apparatus capable of forming a high-quality image.
To achieve the above object, a first aspect of the present invention is a semiconductor laser driving apparatus that drives a semiconductor laser that emits laser light for optical scanning, the apparatus including a light amount detecting section which detects a light amount of a laser light emitted from the semiconductor laser,
a temperature detecting section which detects a laser section temperature, which is one of a temperature of a light-emitting section of the semiconductor laser and a temperature of a vicinity of the semiconductor laser, a temperature adjusting section which adjusts the laser section temperature, a light amount control section which, while the semiconductor laser is driven in a manner the same as for actual optical scanning, controls driving of the semiconductor laser such that a light amount detected by the light amount detecting section becomes equal to a predetermined target light amount, and a temperature deciding section that, while control is performed by the light amount control section, causes the laser section temperature to be altered by the temperature adjusting section, and, if a light amount detected by the light amount detecting section is within a predetermined error span, decides on the laser section temperature as a setting temperature for optical scanning of the semiconductor laser. The light amount detecting section includes all photoelectric-conversion devices such as a photodiode or phototransistor and the temperature detecting section includes all temperature sensors such as a thermistor or thermocouple. Moreover, the temperature adjusting section includes all devices capable of performing at least one of temperature rise and temperature reduction, such as a power transistor, fan, or Peltier device.
Moreover, according to the first aspect of the present invention, driving of the semiconductor laser is controlled such that the light amount detected by the light amount detecting section becomes equal to a predetermined target light amount, while the semiconductor laser is driven by a light amount control section in the same manner as for actual optical scanning. That is, APC is performed while the semiconductor laser is driven by the light amount control section in the same manner as for actual optical scanning.
Furthermore, in the first aspect of the present invention, while control is performed by the light amount control (that is, while APC is performed), the temperature of the light-emitting section of the semiconductor laser or the temperature near the semiconductor laser is changed by the temperature adjusting section. When a light amount changes detected by the light amount detecting section are kept within a predetermined error span, the temperature of the light-emitting section of the semiconductor laser or the temperature near the semiconductor laser is decided on as a setting temperature for optical scanning by the semiconductor laser. The above error span can be, for example, a range outside which visible irreguralities occur in a recorded image or image formed from read image information (for example, s range of error from a target light amount from xe2x88x920.5% up to +0.5%). It is also possible to use an optional range other than the above range, in accordance with required image quality or apparatus specifications.
Thus, the semiconductor laser driving apparatus of the first aspect of the present invention, while driving the semiconductor laser in the same manner as for actual optical scanning, controls driving of the semiconductor laser such that the light amount of laser light emitted from the semiconductor laser becomes equal to the predetermined target light amount, changes the temperature of the light-emitting section of the semiconductor laser or the temperature near the semiconductor laser and thus decides on the temperature of the light-emitting section of the semiconductor laser or the temperature near the semiconductor laser when the above light amount changes are kept within the predetermined error span as the setting temperature of the semiconductor laser under optical scanning. Therefore, it is possible to determine the setting temperature under the same conditions as for actual optical scanning and assuredly prevent the influence of mode-hopping noise, and moreover it is possible to perform the above control and decide the above setting temperature using various combinations of devices for performing APC and ATC that are conventionally provided for a semiconductor laser driving apparatus. Therefore, the apparatus is not increased in size and cost is not increased.
A semiconductor laser driving apparatus of a second aspect of the present invention is the first aspect in which, in a case in which the optical scanning is optical scanning for forming an image on the basis of optical data, while the semiconductor laser is driven such that an image can be formed in accordance with predetermined image data, the light amount control section controls the light amount of the semiconductor laser such that the light amount becomes equal to the target light amount, and the temperature deciding section causes the laser section temperature to be altered, and decides, as the setting temperature for the optical scanning of the semiconductor laser, a laser section temperature level at which an error proportion is at most a predetermined proportion, the error proportion being a ratio of a number of image line light amounts, among a plurality of image line light amounts which are detected by the light amount detecting section at each temperature level, that are outside the predetermined error span, to a total number of lines.
According to second aspect of the present invention, when the optical scanning in the first aspect is optical scanning for forming an image in accordance with image data, the light amount of the semiconductor laser is controlled by the light amount control section so as to become equal to the target light amount, while the semiconductor laser is driven so as to be able to form an image in accordance with the predetermined image data. The above predetermined image data can use image data actually used for optical scanning, image data representative of data likely to be used for optical scanning, average image data likely to be used for optical scanning, gray image data for when the semiconductor laser driving apparatus will administrate color images, or the like.
Moreover, in the second aspect of the present invention, the temperature of the light-emitting section of the semiconductor laser or the temperature near the semiconductor laser is changed by the temperature deciding section, and the setting temperature is the temperature of the semiconductor laser or near the semiconductor laser at which the proportion, with respect to the total number of lines, of line light amounts that are outside the predetermined error span is equal to or less than the predetermined proportion. The above predetermined proportion can use, for example, a proportion at which visible irreguralities occur in a formed image when that proportion is. However, it is also possible to use any proportion in accordance with required image quality or apparatus specifications besides the above proportion.
Thus, according to the second aspect of the present invention, it is possible to obtain the same advantages as the first aspect of the present invention. Moreover, when the optical scanning is optical scanning for forming an image in accordance with image data, the semiconductor laser is controlled such that the light amount of the semiconductor laser becomes equal to the target light amount while the semiconductor laser is driven so as to be able to form an image in accordance with predetermined image data, the temperature of the light-emitting section of the semiconductor laser or the temperature near the semiconductor laser is changed, and the temperature of the light-emitting section of the semiconductor laser near the semiconductor laser is set as the setting temperature for the optical scanning by the semiconductor laser when the ratio of the number of line light amounts outside the predetermined error span to the total number of lines is equal to or less than a predetermined ratio, for a plurality of light amounts of image lines detected at each different temperature. Therefore, compared to the case in which the setting temperature is decided in accordance with a light amount change, it is possible to decide on a setting temperature conforming to more realistic conditions for forming an image with high accuracy.
Automatic temperature control (ATC) is not generally performed with direct detection of the temperature of the light-emitting section of a semiconductor laser, but the temperature near the semiconductor laser is detected by a temperature detecting section such as a thermistor. Therefore, an error may occur between the temperature detected by the temperature detecting section and the temperature of the semiconductor laser, due to a change of ambient temperature of the semiconductor laser. Thus, an error or hunting (fluctuation) or the like may occur in control accuracy of ATC.
Therefore, to more assuredly prevent the influence of mode-hopping noise, it is preferable to determine as the above setting temperature a temperature in a temperature range of the light-emitting section of the semiconductor laser or near the semiconductor laser which temperature range is wider than a predetermined range, over which temperature range the ratio to the total number of lines of the number of light amounts out of the predetermined error span is equal to or less than the predetermined ratio. The above predetermined range may be, for example, a range of xc2x10.5xc2x0 C. from a certain temperature, that is, a range of 1xc2x0 C.
Moreover, in a third aspect of the present invention, it is preferable that in the second aspect of the present invention, when a range of the laser section temperature, in which range the error proportion is at most the predetermined proportion, is broader than a predetermined range, the temperature deciding section decides on one of a temperature at a middle of the range and a temperature near the middle of the range as the setting temperature. That is, as a specific example, when the ratio of the number of line light amounts out of the predetermined error span to the total number of lines is equal to or less than the predetermined ratio for a range of 35xc2x0 C. to 37xc2x0 C., the setting temperature is set to the central temperature of 36xc2x0 C. Thus, it is possible to perform high-accuracy ATC which is not easily influenced by the aforementioned errors or an environmental change or the like.
Furthermore, in a fourth aspect of the present invention, it is allowed that, the temperature deciding section decides, as the setting temperature, a laser section temperature level at which the error proportion is at most the predetermined proportion, the laser section temperature level differing from another laser section temperature level, at which the error proportion is more than the predetermined proportion, by more than a predetermined temperature difference. That is, as a specific example, if temperatures at which the ratio of the number of line light amounts outside the predetermined error span to the total number of lines is larger than a predetermined rate are 34xc2x0 C. and 38xc2x0 C., it is preferable to use as the setting temperature a temperature separate from these temperatures by a predetermined temperature difference such as 1xc2x0 C. or more; that is, any temperature in the range from 35xc2x0 C. to 37xc2x0 C.
When applying the semiconductor laser driving apparatus of the present invention in an image-forming apparatus for forming an image on a photosensitive material with a laser beam emitted from a semiconductor laser, a preferable light amount to be emitted from the semiconductor laser differs for each type of photosensitive material, because sensitivity of a photosensitive material depends on type. Therefore, if the image-forming apparatus can use a plurality of types of photosensitive material, it is possible to form a high-quality image irrespective of the type of photosensitive material, by changing the target light amount in accordance with the type of photosensitive material.
To correspond to the above case, a semiconductor laser driving apparatus of a fifth aspect of the present invention performs the control by the light amount control section and the decision of the setting temperature by the temperature deciding section for a plurality of target light amounts. Thus, even if target light amounts are changed according to requirements, it is possible to assuredly prevent the influence of mode-hopping noise by using a setting temperature corresponding to a current target light amount.
When using the semiconductor laser driving apparatus of the present invention as an apparatus for handling a color image, a plurality of semiconductor lasers for emitting laser beams having wavelengths different from each other are provided.
To correspond to the above case, a semiconductor laser driving apparatus of a sixth aspect of the present invention performs the control by the light amount control section and the decision of the setting temperature by the temperature deciding section for the plurality of semiconductor lasers emitting laser light of different wavelengths. Thus, it is possible to assuredly prevent the influence of mode-hopping noise for each of the semiconductor lasers.
A seventh aspect of the present invention is a semiconductor laser driving method for driving a semiconductor laser for emitting a laser beam for optical scanning. The present invention controls driving of the semiconductor laser such that, while the semiconductor laser is driven in a manner the same as for actual optical scanning, light amount of the laser light emitted from the semiconductor laser is a predetermined target light amount; and, during control of driving, alters temperature of one of a light-emitting section of the semiconductor laser and a vicinity of the light emitting section and, when the light amount of the laser light is within a predetermined error span, decides on the temperature of the one of the light-emitting section of the semiconductor laser and the vicinity of the light emitting section as a setting temperature for optical scanning of the semiconductor laser.
Therefore, because the semiconductor laser driving method of the seventh aspect of the present invention functions similarly to the invention of the first aspect, it is possible to determine the setting temperature under the same conditions as for actual optical scanning and assuredly prevent the influence of mode-hopping noise. Moreover, because the control and the decision of the setting temperature can be performed using various combinations of units for performing APC and ATC that are conventionally provided for a semiconductor laser driving apparatus, the apparatus is not increased in size and cost is not raised.
Further, an image-forming apparatus of an eighth aspect of the present invention includes a semiconductor laser driving apparatus according to the first aspect, a semiconductor laser to be driven by the semiconductor laser driving apparatus, a temperature control section for controlling the temperature adjusting section such that a temperature detected by a temperature detecting section becomes equal to a setting temperature decided by the temperature deciding section, and a recording medium on which an image will be formed by laser light emitted from the semiconductor laser.
According to the image-forming apparatus of the eighth aspect of the present invention, when the semiconductor laser is driven by the semiconductor laser driving apparatus, the temperature adjusting section is controlled by the temperature control section such that a temperature detected by the temperature detecting section becomes equal to the setting temperature decided by the temperature deciding section and, in the above state, the image is formed on the recording medium by the laser light emitted from the semiconductor laser.
Because the image-forming apparatus of the eighth aspect of the present invention controls the temperature of the light-emitting section of the semiconductor laser or the temperature near the semiconductor laser such that the temperature becomes equal to the setting temperature determined by the temperature deciding section of the semiconductor laser driving apparatus of the present invention while the image is being formed, it is possible to form a high-quality image in which the influence of mode-hopping noise is prevented.
As described above in detail, according to the semiconductor laser driving apparatus and a semiconductor laser driving method of the present invention, driving of a semiconductor laser is controlled such that light amount of laser light emitted from the semiconductor laser becomes equal to a predetermined target light amount while the semiconductor laser is driven in the same way as for actual optical scanning and, while the control is performed, the temperature of the light-emitting section of the semiconductor laser or the temperature near the semiconductor laser is changed and the temperature when light amount changes in the above case are kept in a predetermined error span is decided on as a setting temperature for the optical scanning by the semiconductor laser. Therefore, it is possible to determine the setting temperature in conditions the same as for actual optical scanning and to assuredly prevent the influence of mode-hopping noise. Moreover, advantages that the apparatus is not increased in size and costs are not raised are obtained because the above control and determination of the setting temperature can be performed using various combinations of units for performing APC and ATC that are conventionally provided for a semiconductor laser driving apparatus.
Moreover, an image-forming apparatus of the present invention controls the temperature of the light-emitting section of a semiconductor laser or the temperature near the semiconductor laser so as to become equal to a setting temperature decided by a temperature deciding section of the semiconductor laser driving apparatus of the present invention. Therefore, an advantage can be obtained that it is possible to form a high-quality image in which the influence of mode-hopping noise is prevented.